Microwave drying system

ABSTRACT

Apparatus for drying moisture-laden dielectric material including a pair of substantially parallel slotted waveguides positioned side by side, material to be dried travelling through the slots of these waveguides, and microwave generating means connected to one end of one of these waveguides and to the opposite end of the other of the waveguides so that microwave energy is directed in opposite directions side by side through the material to effect even drying thereof across its width.

United States Patent [191 Mathur 1 Jan.9, 1973 [s41 MICROWAVE DRYING SYSTEM [75] Inventor: Vishwa N. P. Mathur, Vancouver,

B.C.,Canada [73] Assignee: MacMlllan Bloedel Limited, Vancouver, British Columbia, Canada [22] Filed: v June3,197l

[21] App1.No.: 149,448

[52] US. Cl ..219/l0.55, 34/1 [51] Int. Cl. ..H05b 9/06 [58] Field of Search ..2l9/l0.55; 34/1 [56] References Cited UNITED STATES PATENTS 3,666,905 5/1972 Muller et al ..219/l0.55 3,209,113 9/1965 Verstraten et al ....2l9/10.55 3,475,577

10/1969 Gade et al. ..219/10.55

TO WA TE R LOA D 2/1966 Timmermans et a1 ..2l9/10.55 2/1967 Timmermans et a1 ..219/10.55

Primary Examiner-R. F. Staubly Assistant Examiner-Hugh D. Meger Attorney-Fetherstonhaugh & Co.

[57] ABSTRACT Apparatus for drying moisture-laden dielectric material including a pair of substantially parallel slotted waveguides positioned side by side, material to be dried travelling through the slots of these waveguides, and microwave generating means connected to one end of one of these waveguides and to the opposite end of the other of the waveguides so that microwave energy is directed in opposite directions side by side through the material to effect even drying thereof across its width.

6 Claims, 2 Drawing Figures PATENTEDJAN 9197s 3.710.064

ENERGY GRAPH ENERGY ENERGY ENERGY GRAPH Mil/EMT VISHWA N. P. MATHUR ATTMNE s MICROWAVE DRYING SYSTEM This invention relates to a microwave drying system for moisture-laden dielectric materials, such as lumber, wood based materials, bagasse, paper and the like.

The use of slotted meandering waveguides for drying materials is well known. In one prior theoretical use of slotted waveguides, a plurality of these guides are aligned so that material to be dried passes successively through them. As is well known, the maximum amount of microwave energy is available at the beginning of each waveguide and becomes less in each transverse section or leg thereof. In the prior waveguide setup, the first transverse section of each succeeding guide is adjacent the last transverse section of the previous guide. In other words, at adjacent ends of two waveguides, minimum microwave energy is flowing in one direction and maximum microwave energy is flowing in the opposite direction, subjecting the material to uneven stresses.

At the beginning of each waveguide system of the prior art, the maximum microwave power is directed into the material at one side thereof so that there is maximum drying at that side and the drying tapers off very rapidly towards the other side. This may not be harmful for some materials, but it is damaging for wood and, in fact, has made microwave drying in the past for wood impractical. With a conventional slotted single meandering waveguide system, the material first contacts the waveguide at the end closest to the generator. Most of the energy from the generator is absorbed in the first transverse section of the guide so that the wet material is heated and dried very rapidly, with consequent damage.

The microwave drying system of the present invention overcomes the above-noted disadvantages and makes microwave drying effective for drying wood. In this system beams of microwave energy are directed side by side through the material from opposite sides thereof so that in any given area of the material the maximum energy is applied from opposite sides, resulting in the material being heated substantially evenly from opposite sides across the width thereof. In addition, it is preferred to use slotted meandering waveguides and to direct the material first into the end of one of the waveguides which is remote from the end thereof connected to the microwave source. In this way, microwave energy is applied to the material in successive increasing amounts until it reaches the inlet end of the waveguide where it encounters the maximum microwave energy,. When wood or wood based products are dried in this way, they are heated slowly enough so that checking and other loss of grade defects are eliminated or at least greatly reduced. The temperature builds up gradually in the wood, and in the area where the maximum microwave energy is applied thereto, said energy flows in opposite directions side by side through the wood so that the drying effect is substantially uniform across the width thereof.

A microwave drying system for moisture-laden dielectric material in accordance with the present invention comprises means for directing substantially parallel beams of microwave energy side by side through the dielectric material to be dried from opposite sides thereof and in opposite directions to dry said material substantially uniformly across the width thereof. The preferred drying system includes means for directing a plurality of additional beams of microwave energy across the dielectric material alternately from opposite sides of the material, this arrangement being such that the beams increase in intensity up to the first-mentioned microwave beams.

An example of a preferred form of drying apparatus in accordance with this invention is illustrated in the accompanying drawings, in which FIG. 1 is an isometric diagrammatic view of drying apparatus including two meandering waveguides, and

FIG. 2 diagrammatically illustrates the main drying action in the material being dried.

Referring to FIG. 1 of the drawings, 10 is microwave drying apparatus in accordance with this invention including two slotted meandering waveguides l2 and 13 arranged in succession, but reversed relative to each other. These are standard waveguides. Guide 12 has a transverse main or entrance section 16 followed by a plurality of transverse sections 17 interconnected at adjacent ends by curve sections 18 in the usual manner. The opposite end or outlet section 20 is connected to a suitable water load or other microwave absorption means, not shown. The waveguide sections 16, 17 and 20 have the usual aligned slots 22 therein through which the material to be dried travels, which in this example is lumber 25.

Similarly, waveguide 13 has a transverse entrance section 28 at one end followed by a plurality of transverse sections 29 interconnected by curved sections 30. The outlet section 32 of this waveguide is connected to a waterload or other suitable microwave absorption means, not shown. The transverse sections of waveguide 13 have the usual aligned slots 34 therein, which are aligned with slots 22 of guide 12, and through which the material or lumber 25 travels. The most important thing about this invention is that the entrance sections 16 and 28 of waveguides l2 and 13 are positioned as closely as possible side by side. The inlet ends 35 and 36 of guides 12 and 13, respectively, are located on opposite sides of the path alongwhich the lumber travels. 1

Suitable microwave generating means is connected to the inlet ends 35 and 36 of the waveguides. The generating means or source may consist of two separate microwave generators, but it is preferably to connect them to a single microwave generator 40.-This ensures the microwave energy being directed to the two waveguides being equal. Generator 40 is connected to the waveguides by any suitable means, and these are known in the industry. In this example, the microwave energy from the generator is divided evenly into two parts by what is known as a magic T 42. As this is well known in the industry it does not require further description herein. The magic T is connected by tubular waveguides 44 and 45 to the inlet ends 35 and 36 of waveguides l2 and 13.

During operation of drying apparatus 10, the lumber 25 or other material, is directed through the respective slots 22 and 34 of the two waveguides l2 and 13. The lumber enters the apparatus through the slotin the outlet section 20 of guide 12. The lumber emerges from slot 22 of the entrance section 16 of guide 12 and enters slot 34 of the entrance section 28 of guide 13, leaving the apparatus through the slot of outlet end 32 of the latter guide.

As the lumber passes through the adjacent sections 16 and 28 of the waveguides, it is subjected to beams of microwave energy from generator 40 travelling in opposite directions across the width of the board. With this arrangement, any given portion of the board 'is simultaneously subjected to the maximum amount of microwave energy divided into two parts and travelling in opposite directions through the board. This balances the energy input across the width of the lumber and, therefore, it is dried substantially evenly across its width. Thus, each section or portion of the board is subjected to substantially'the same forces from side to side thereof so that not only is the drying quite even, but checking is greatly reduced.

It is known that most of the energy from the generator is absorbed in the first crossing of the waveguide. Thus, the wood is subjected to the highest drying eflect in an area spaced from the outlet section 20 of waveguide 12. As a result, when each board enters the system at outlet section 20, it is subjected to the minimum amount of microwave power or energy, and this power or energy is successively increased as the board travels through the succeeding transverse sections 17 of the guide.'ln other words, the material is pre-warmed before it reaches the waveguide inlet sections 16 and 28 where it is subjected to the maximum power or energy. This helps to reduce the checking in the wood which would occur if the moist wood were instantly subjected to the relatively high temperatures.

Not all of the microwave energy entering waveguide 13 is absorbed by the wood at entrance section 28, but the following transverse sections 29 and 32 direct a substantial portion of the remaining power into the wood.

FIG. 2 graphically illustrates the main application of the microwave energy in this apparatus. In this diagram, 50 represents the material, such as lumber, being dried. Energy in waveguide sections 16 and 28 is directed into and through the wood from opposite sides, as indicated by arrows 51 and 52, respectively. Graph 54 shows how the maximum amount of energy from 51 is absorbed near edge 55 of the board, while graph 58 illustrates how the maximum amount of energy from 52 is absorbed near edge 59 of the board. However, the graphs also illustrate that the amount of energy from the two beams gradually decreases towards the opposite edges'of the board so that each edge of the board absorbs a maximum amount of energy from one beam and a minimum amount of energy from the other beam. As a result, the energy absorbed is fairly even across the width of the board. This absorption of energy represents the drying effect within the board.

lclaim l. A microwave drying system for moisture-laden dielectric material comprising two substantially parallel tubular waveguides arranged side by side, said waveguides having aligned slots in walls thereof and extending longitudinally of the respective waveguides and through which dielectric material can move along a path of travel in a direction longitudinally of the material, said material having side edges facing opposite ends of the waveguides, means for directing a beam of microwave energy into one end of one of said waveguides and against one of said edges of the dielectric material on one side of the paths of travel, and means for directing another beam of microwave eneagg into an opposite end of the other of said wavegui and against the other of said edges of dielectric material on an opposite side of the path of travel, whereby beams of microwave energy simultaneously travel in the waveguides in opposite directions through said material from the opposite edges of the material as the latter travels through the waveguides.

2. A microwave drying system as claimed in claim 1 including microwave energy absorption means connected to the end of each waveguide opposite to the end of said each waveguide to which one of the microwave beam directing means is connected.

3. A microwave drying system as claimed in claim 1 in which the means for directing microwave energy into the ends of the two waveguides comprises a single generator, and a magic I T connected to divide microwave energy from the generator into two parts and to direct said parts to said ends of the waveguides.

4. 'A microwave drying system comprising two slotted meandering waveguides arranged in close succession and reversed relative to each other, each of said waveguides having a plurality of interconnected transverse sections and aligned slots in said sections extending longitudinally of the sections and through which dielectric material can move along a path of travel through both waveguides in a direction longitudinally of the material, said material having opposite side edges facing laterally out from the path of travel, each of said waveguides having an inlet end and an outlet end, said waveguides being arranged with the respective inlet ends thereof positioned on opposite sides of said path of travel, means for directing a beam of microwave energy into'the inlet end of one of said waveguides and against one of said edges of the dielectric material on one side of the path of travel, and means for directing another beam of microwave energy into the inlet end of the other of said waveguides and against the other of said edges of the dielectric material on the opposite side of said path of travel, whereby beams of microwave energy simultaneously travel in the waveguides in opposite directions through said material from the opposite edges of the material as the latter travels through the waveguides.

5. A microwave drying system as claimed in claim 4 including microwave energy absorption means connected to the outlet end of each waveguide.

6. A microwave drying system as claimed in claim 4 in which the means for directing microwave energy into the inlet ends of the two waveguides comprises a single generator, and a magic T connected to divide microwave energy from the generator into two parts and to direct said parts to said inlet end of the waveguides.

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1. A microwave drying system for moisture-laden dielectric material comprising two substantially parallel tubular waveguides arranged side by side, said waveguides having aligned slots in walls thereof and extending longitudinally of the respective waveguides and through which dielectric material can move along a path of travel in a direction longitudinally of the material, said material having side edges facing opposite ends of the waveguides, means for directing a beam of microwave energy into one end of one of said waveguides and against one of said edges of the dielectric material on one side of the paths of travel, and means for directing another beam of microwave energy into an opposite end of the other of said waveguides and against the other of said edges of dielectric material on an opposite side of the path of travel, whereby beams of microwave energy simultaneously travel in the waveguides in opposite directions through said material from the opposite edges of the material as the latter travels through the waveguides.
 2. A microwave drying system as claimed in claim 1 including microwave energy absorption means connected to the end of each waveguide opposite to the end of said each waveguide to which one of the microwave beam directing means is connected.
 3. A microwave drying system as claimed in claim 1 in which the means for directing microwave energy into the ends of the two waveguides comprises a single generator, and a magic T connected to divide microwave energy from the generator into two parts and to direct said parts to said ends of the waveguides.
 4. A microwave drying system comprising two slotted meandering waveguides arranged in close succession and reversed relative to each other, each of said waveguides having a plurality of interconnected transverse sections and aligned slots in said sections extending longitudinally of the sections and through which dielectric material can move along a path of travel through both waveguides in a direction longitudinally of the material, said material having opposite side edges facing laterally out from the path of travel, each of said waveguides having an inlet end and an outlet end, said waveguides being arranged with the respective inlet ends thereof positioned on opposite sides of said path of travel, means for directing a beam of microwave energy into the inlet end of one of said waveguides and against one of said edges of the dielectric material on one side of the path of travel, and means for directing another beam of microwave energy into the inlet end of the other of said waveguides and against the other of said edges of the dielectric material on the opposite side of said path of travel, whereby beams of microwave energy simultaneously travel in the waveguides in opposite directions through said material from the opposite edges of the material as the latter travels through the waveguides.
 5. A microwave drying system as claimed in claim 4 including microwave energy absorption means connected to the outlet end of each waveguide.
 6. A microwave drying system as claimed in claim 4 in which the means for directing microwave energy into the inlet ends of the two waveguides comprises a single generator, and a magic T connected to divide microwave energy from the generator into two parts and to direct said parts to said inlet end of the waveguides. 